In 1924 British Patent 250,991 first disclosed the segregation roast as a means to recover copper from unamenable oxide and silicate ores. Since that time, substantial technical efforts have been expended to make the process commercially viable. In the period from 1930 to 1970 a number of commercial operations used the process. The United States Bureau of Mines made extensive efforts to advance the process, and these efforts were reported in Reports of Investigations (RI5501; 1959; RI 5826, 1961; RI 6215, 1963) and resulted in a commercial operation in the United States. In the 1960's and 1970's attempts were made to extend the process to the recovery of nickel and cobalt from laterite deposits. An example of this is disclosed in Canadian Patent 48,377 of 1970.
While these efforts were a response to the demonstrated need to find a means of recovering metals from unamenable ores, the process is no longer in commercial use because it was proven to be very complex and energy intensive. The recovery of copper required an operating temperature of 750° C. to 800° C. and the recovery of nickel required an even higher temperature of 850° C. to 900° C.
As first developed, the reactions of the segregation roast utilized much less than stoichiometric amounts of chloride ion which was converted to hydrogen chloride gas that hydrochlorinated the desired metal oxides, silicates, or carbonates. The metal chlorides had sufficient volatility to migrate to carbon particles. Metal reduction associated with the carbon particles resulted in metal beads and the regeneration of the hydrogen chloride to hydrochlorinate additional metal oxide. The result was a “segregation” of the metal beads from the roasted gangue. The isolation of the metallic particles is then undertaken by means such as gravity separation, magnetic separation, electrostatic separation and even chemical separations specific to the desired metals.
Improvements to the segregation roast process have been directed to oxidic materials. At this time however, there is great concern about sulfides which are contained in mine or concentrator tailings. These tailings may originate from metal mining activity but may equally originate in coal mining activities. In metal mining and concentrating, pyrite (iron sulfide) is a frequent impurity. Its high iron and sulfur contents are unwanted in the smelter and so they are discarded along with substantial values in gold, silver, copper, zinc, nickel and cobalt that are often included in the waste and not recovered. Similarly, in order to meet required sulfur levels, coal is frequently processed to remove unwanted pyrite and ash formers. This results in “gob” piles that represent a loss of coal and an environmental hazard. Therefore, there is a need for a low cost and environmentally benign process to treat pyrite and other metal sulfide sources.
The segregation roast involves two interrelated reactions. One is the hydrochlorination of certain metal compounds which have sufficient volatility to migrate to nearby carbon surfaces. On the carbon surfaces, the second reaction occurs which is the reaction of water to reform the hydrogen chloride, with the carbon being oxidized and leaving the reaction site. The exact mechanism of the reaction has not been established but it has been postulated that water reacts with carbon in the well known “water gas” reaction to produce hydrogen which reduces the chloride to metal and regenerates the hydrogen chloride. The carbon monoxide of the water gas reaction may also play a role in reducing metal oxide formed by the hydrolysis of the metal chloride. This results in the carbon with metal chloride on it reacting to become a gas, and leaving behind a metal. The metal that remains is often, but not always, in hollow spheres which are holed where the gas escaped.
These reactions are attractive because they avoid the limiting factors in the direct reduction of metals below their melting points. That is, the tendency for the metal to form on the surface of the metal compound and thereafter limit the rate of metalization by the necessity for the reduction byproduct, water or carbon oxide to escape by a process of diffusion through the metal, which is necessarily a slow and energy expensive process. The segregation process, which has the metal-forming reaction occurring at a different site on the carbon particle than where the carbon oxidation reaction is occurring, avoids this difficulty.
Similarly, in the oxidation of sulfides, the reaction is hindered by the formation of sulfur on the sulfide mineral surface resulting in a diffusion-limited process below the boiling point of sulfur. Where oxygen is the oxidant, there is a strong tendency for the sulfur to burn above its boiling point and to form sulfur dioxide. Indeed, this is the standard product of sulfide roasting. As in the reduction reaction of the segregation roast, the use of much less than stoichiometric amounts of hydrogen chloride and the unique properties of microwave radiation permits the production of elemental sulfur and metal chlorides without the diffusion limitation.
U.S. Pat. Nos. 4,311,520 and 4,324,582 teach the use of microwave radiation as a means of selectively energizing copper, nickel and cobalt in their oxidic and sufidic compounds without substantially heating the gangue material present. The conversion of oxidic materials to chlorides is also shown. Neither patent discloses the use of carbon in the microwave for converting to metal, which is the essence of the segregation process.
U.S. Pat. No. 5,698,759 teaches that waste polyvinyl chloride can be an effective source of both chloride and energy at high temperatures. U.S. Pat. No. 5,972,302 shows that pyritic sulfides can be effectively converted to sulfur and metal oxides in the microwave. The beneficial role of carbon and chlorides is not recognized in this patent.
U.S. Pat. No. 4,906,290 teaches the use of carbothermic reduction in a microwave as a precursor to smelting. It teaches the facts of the particularly strong receptivity of carbon for microwave radiation and the high temperatures that can be attained. The patent cites heating pyrite to a temperature of releasing its labile sulfur and then carbothermically reducing iron oxide to carbon. The problem with the carbothermic iron process is that iron forms on the surface of the iron oxide particle after which time, a very slow process of diffusion of oxygen through the iron is required. In U.S. Pat. No. 4,906,290 this problem is overcome by recourse to a second very high temperature melting furnace.